This application is based on patent application No. 11-226174 filed in Japan, the contents of which are hereby incorporated by references.
This invention relates to a measuring apparatus for measuring a characteristic of an object, and particularly to an apparatus for measuring a color of a metallic coating and a pearl-color coating.
In a metallic coating and a pearl-color coating used as a coating of an automotive vehicle, bright materials 101 comprised of thin pieces of aluminum or mica is contained in a film 102 as shown in FIG. 13. Since the orientation of the bright materials 101 varies as shown in FIG. 13, the intensity of light reflected by the bright materials 101 differs depending upon a viewing direction, which provides a metallic effect and a pearl effect. Multi-angle calorimeters of the type in which illumination light is projected in one direction and detected in a multitude of directions or illumination light is projected in a multitude of directions and detected in one direction are used as calorimeters for measuring the color of the metallic coating and the pearl-color coating having such a characteristic.
Conventionally, multi-angle calorimeters whose light detecting direction is mechanically changed include those of the type in which an illumination light is projected in one direction and detected in a multitude of directions, but those of the type in which illumination lights are projected in a multitude of directions and detected in one direction are widely used as portable multi-angle calorimeters used on the spot since they have a high reliability because not having a movable portion and have a shorter measurement time.
A known multi-angle calorimeter of the type in which illumination light is projected in a multitude of directions and detected in one direction is provided with a light detector 104 for detecting reflected right in a direction at 45xc2x0 to a normal to a surface of a measurement object 103 and three illuminator 106, 107 for illuminating the measurement object 103 in three directions as shown in FIG. 14. In such a multi-angle calorimeter, illuminating directions by the respective illuminators are generally based on a regular reflection direction of the light detecting direction, i.e., a direction (S in FIG. 14) symmetrical with the light detecting direction with respect to the normal to the object surface and expressed by angles with a side where the normal to the object surface is located being positive. One illuminating direction is set, for example, at 45xc2x0, i.e., the normal direction (center axis of a calorimeter main body) to the object surface (illuminator 106), the second illuminating direction is set, for example, at 15xc2x0 closer to the regular reflection direction of the light detecting direction (illuminator 105), and the last illuminating direction is set, for example, at 110xc2x0 closer to the light detecting direction (illuminator 107). Lights reflected by the measurement object 103 when the measurement object 103 is illuminated in the respective illuminating directions are detected by the light detector 104, and a reflection characteristic and a color value are obtained based on an amount of the detected light, and characteristics of the measurement object are expressed by the reflection characteristic and the color value in the respective illuminating directions. Generally, illuminating directions closer to the regular reflection direction of the light detecting direction are referred to as highlight directions, whereas those closer to the light detecting direction are referred to as shade directions.
The bright materials 101 shown in FIG. 13 are so arranged as to be substantially parallel to an outer surface of the film 102 (a normal 101in to the bright materials 101 and a normal 102n to the outer surface of the film 102 are substantially parallel to each other). If an angle between the normal 101n and the normal 102n is assumed to be t as shown in FIG. 13, an angle distribution P(t) displays a characteristic approximate to a normal distribution having a peak at t=0 as shown in FIG. 15. Accordingly, an angle distribution of the reflected light from the bright materials when a metallic coating or a pearl-color coating is illuminated in a certain direction also approximates to a normal distribution having a peak in a regular reflection direction (i.e., direction symmetrical with the illuminating direction with respect to a normal to the film surface) by the film surface.
When a reflection characteristic R(x) is measured by illuminating the measurement object in various directions, it contains components by diffused reflected light Ld having no angle dependency and those by regularly reflected light from the surface of the measurement object in addition to those by the reflected light from the bright materials. In the illumination in the shade direction distant from the regular reflection direction of the light detecting direction, there is almost no contribution of the reflected light from the bright materials, and the reflection characteristic moderately changes upon a change in the angle of the illuminating direction. On the other hand, in the illumination in the highlight direction close to the regular reflection direction of the light detecting direction, there is a significantly large contribution of the reflected light from the bright materials since the reflection surfaces of the bright materials are mostly substantially parallel to the object surface as shown in FIG. 15, and the reflection characteristic steeply changes upon a change in the angle of the illuminating direction. In the illumination in the direction at 45xc2x0 to the light detecting direction, the reflection characteristic displays an intermediate characteristic between the above two cases.
Since the reflection characteristic by the illumination in the highlight direction is particularly essential in understanding the property of the bright materials representing the features of the metallic coating and the pearl-color coating, it is desired to obtain this reflection characteristic with high precision. However, the value of the reflection characteristic largely changes even upon a slight change in the angle of the illuminating direction. For a high-precision measurement, it is necessary to precisely set the orientation of the multi-angle calorimeter with respect to the surface of the measurement object, i.e., to precisely coincide the center axis of the main body and the normal to the surface of the measurement object.
For example, in the case that a coating surface on a body of an automotive vehicle is measured by a portable colorimeter, a contact surface of the calorimeter is formed by rubber or like material so as not to damage the coating surface. Accordingly, it has not been easy to precisely coincide the center axis of the main body and the normal to the object surface. Particularly, it has been even more difficult when the surface of the measurement object is curved as the body of the automotive vehicle is.
In order to overcome the above problem, calorimeters having a function of adjusting an angle of the main body with respect to the object surface have been put into practice. However, there are still problems of an increased number of parts and a complicated construction because a sensor for detecting an angle is necessary, and also a problem of a long measurement time since time is required for angle adjustment in each measurement.
It is an object of the present invention to provide a measurement apparatus which is free from the problems residing in the prior art.
It is another object of the present invention to provide a color measurement apparatus which enables a high precision color measurement by correcting a measurement result according to an angle of inclination of a main boy of the apparatus with respect to a surface of an object.
It is another object of the present invention to provide a color measurement apparatus which can precisely obtain a refection characteristic of a highlight direction even if an apparatus main body is slightly inclined with respect to an object surface.
The invention is directed to a measurement apparatus for measuring color of an object is provided with a main body having an opening opposed to an object to be measured.
According to an aspect of the invention, a measurement apparatus is provided with a plurality of illuminators for illuminating a surface of the object in directions different from one another, a light detector for detecting reflected light in a specified direction from the object illuminated by the plurality of illuminators and outputting light detection signals corresponding to light intensities. In the apparatus, further, there are provided a first calculator for calculating reflection characteristic measurement values of the measurement object in correspondence with the plurality of illuminators based on the light detection signals, a storage device for storing an approximate function having an angle of an illuminating direction with respect to a reference direction as a variable if the reference direction is a direction symmetrical with the specific direction with respect to a center axis of the main body in parallel to a normal to the opening, and having a plurality of undetermined coefficients including an angle of inclination of the center axis of the main body with respect to a normal to the surface of the object, and a second calculator for determining a plurality of undetermined coefficients based on the respective reflection characteristic measurement values and the angles of the illuminating directions, and correcting the respective reflection characteristic measurement values using the approximate function whose undetermined coefficients are determined.
According to another aspect of the invention, a measurement apparatus is provided with a plurality of illuminators for illuminating a surface of the object in directions different from one another. The plurality of illuminators includes a first illuminator provided at one side of a reference direction where a center axis of the main body is located and a second illuminator provided on the other side of the reference direction if the reference direction is a direction symmetrical with the specific direction with respect to the center axis of the main body in parallel to a normal to the opening. The first and second illuminators are provided in positions symmetrical with each other with respect to the reference direction. The apparatus is further provided with a light detector for detecting reflected light in a specified direction from the object illuminated by the plurality of illuminators and outputting light detection signals corresponding to light intensities, and a calculator for calculating a reflection characteristic of the object corresponding to the first illuminator based on a sum of the light detection signals corresponding to the first and second illuminator.
According to still another aspect of the invention, a measurement apparatus is provided with an illuminator for illuminating a surface of the object in a specific direction, a plurality of light detectors for detecting reflected light in directions different from one another from the object illuminated by the illuminator and outputting light detection signals corresponding to light intensities. Further, there are provided a first calculator for calculating reflection characteristic measurement values of the object in correspondence with the plurality of light detectors based on the light detection signals, a storage device for storing an approximate function having an angle of a light detecting direction with respect to a reference direction as a variable if the reference direction is a direction symmetrical with the specific direction with respect to a center axis of the main body in parallel to a normal to the measurement opening, and having a plurality of undetermined coefficients including an angle of inclination of the center axis of the main body with respect to a normal to the surface of the object, and a second calculator for determining a plurality of undetermined coefficients based on the respective reflection characteristic measurement values and the angles of the light detecting directions, and correcting the respective reflection characteristic measurement values using the approximate function whose undetermined coefficients are determined.
According to further aspect of the invention, a measurement apparatus is provided with an illuminator for illuminating a surface of the object in a specific direction, and a plurality of light detectors for detecting reflected light in directions different from one another from the object illuminated by the illuminator and outputting light detection signals corresponding to light intensities. The plurality of light detectors include a first light detector provided at one side of a reference direction where a center axis of the main body is located and a second light detector provided on the other side of the reference direction if the reference direction is a direction symmetrical with the specific direction with respect to the center axis of the main body in parallel to a normal to the opening. The first and second light detectors are provided in positions symmetrical with each other with respect to the reference direction. The apparatus is further provided with a calculator for calculating a reflection characteristic of the object corresponding to the first light detector based on a sum of the light detection signals corresponding to the first and second light detectors.